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A330707
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a(n) = ( 3*n^2 + n - 1 + (-1)^floor(n/2) )/4.
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4
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0, 1, 3, 7, 13, 20, 28, 38, 50, 63, 77, 93, 111, 130, 150, 172, 196, 221, 247, 275, 305, 336, 368, 402, 438, 475, 513, 553, 595, 638, 682, 728, 776, 825, 875, 927, 981, 1036, 1092, 1150, 1210, 1271, 1333, 1397, 1463
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OFFSET
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0,3
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COMMENTS
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Essentially four odds followed by four evens.
Last digit is neither 4 nor 9.
Essentially twice or twin sequences in the hexagonal spiral from A002265.
21 21 21 22 22 22 22
21 14 14 14 14 15 15 23
20 13 8 8 8 9 9 15 23
20 13 8 4 4 4 4 9 15 23
20 13 7 3 1 1 1 5 9 16 23
20 13 7 3 1 0 0 2 5 10 16 24
19 12 7 3 0 0 2 5 10 16 24
19 12 7 3 2 2 5 10 16 24
19 12 6 6 6 6 10 17 24
19 12 11 11 11 11 17 25
18 18 18 18 17 17 25
.
0, 1, 3, 7, 13, 20, 28, 38, 50, ...
1, 2, 4, 6, 7, 8, 10, 12, 13, ...
1, 2, 2, 1, 1, 2, 2, 1, 1, ... period 4. See A014695.
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LINKS
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FORMULA
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a(1+2*n) + a(2+2*n) = A033579(n+1).
a(40+n) - a(n) = 1210, 1270, 1330, 1390, 1450, ... . See 10*A016921(n).
G.f.: x*(1 + 2*x^2) / ((1 - x)^3*(1 + x^2)).
a(n) = 3*a(n-1) - 4*a(n-2) + 4*a(n-3) - 3*a(n-4) + a(n-5) for n>4.
(End)
E.g.f.: (cos(x) + sin(x) + (-1 + 4*x + 3*x^2)*exp(x))/4. - Stefano Spezia, Dec 27 2019
a(n) = ( 3*n^2 + n - 1 + sqrt(2)*sin((2*n+1)*Pi/4) )/4 = ( 3*n^2 + n - 1 + (-1)^floor(n/2) )/4. - G. C. Greubel, Dec 30 2019
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MAPLE
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seq((3*n^2+n-1+sqrt(2)*sin((2*n+1)*Pi/4))/4, n = 0..60); # G. C. Greubel, Dec 30 2019
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MATHEMATICA
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LinearRecurrence[{3, -4, 4, -3, 1}, {0, 1, 3, 7, 13}, 60] (* Amiram Eldar, Dec 27 2019 *)
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PROG
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(PARI) concat(0, Vec(x*(1 + 2*x^2) / ((1 - x)^3*(1 + x^2)) + O(x^60))) \\ Colin Barker, Dec 27 2019
(Magma) [(3*n^2+n-1+ (-1)^Floor(n/2))/4: n in [0..60]]; // G. C. Greubel, Dec 30 2019
(Sage) [(3*n^2+n-1+(-1)^floor(n/2))/4 for n in (0..60)] # G. C. Greubel, Dec 30 2019
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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